Coating of titanium alloys

ABSTRACT

Titanium and its alloys are provided with a chemically bonded coating derived from a reaction at high temperature of certain aluminum-pigmented silicone resins with the titanium substrate to form in situ by decomposition a quasi-organic complex coating which not only provides improved oxidation and corrosion resistance together with improved physical properties associated with surface-oriented characteristics, such as fatigue and creep stability, but which extends the useful operating range of a number of these alloys to temperatures of 650*-900* F. or higher.

O United States Patent [151 3,640,778

Winfree et al. 1 Feb. 8, 1972 [541 COATING 0F TITANIUM ALLOYS 2,743,1924/1956 White ..117/132 x [72] Inventors: Jules P. Wlnfree, Jupiter,Fla.; Herbert E. g Todd West Hartford Conn." Charles C. m Mcc; PalmBeach Gardehs Fla 3,037,880 6/1962 Hamnk ..l [7/131 X 731 Assignce:United Aircraft Corporation, East l-lart- Primary Emminer-Ralph Kendallford, Conn. Attorney-Richard N. James [22] Filed: Mar. 27, 1969 57ABSTRACT PP' N05 8111071 Titanium and its alloys are provided with achemically bonded coating derived from a reaction at high temperature ofcertain 521 US. Cl. ..148/6 117/132 BS 148/63 alumimm'pigmenmd Siliwnewith the titanium [51] 6 7/00 strate to form in situ by decomposition aquasi-organic com- [58] field of Search ZA plex coating which not onlyprovides improved oxidation and 1 l 4 6 corrosion resistance togetherwith improved physical properties associated with surface-orientedcharacteristics, such a [56] References Cited fatigue and creepstability, but which extends the useful operating range of a number ofthese alloys to temperatures of UNITED STATES PATENTS 650190011; or g2,54l,8l3 2/l95l Frisch et a]. ..1 17/46 7 Claims, 1 Drawing Figure #[47L X/ a)? Viv-(@4760 may) Cl flfcf COATING'OF TITANIUM ALLOYS BACKGROUNDOF THE INVENTION The present invention relates to the coating oftitanium and the titanium alloys, particularly to the coating of thosehighstrength, titanium alloy components having particular utility in gasturbine engine components.

Because of their favorable strength-to-weight ratio, the titanium alloysare extensively used in certain gas turbine engines, particularly invarious compressor components. Unfortunately, however, principallybecause of the high reactivity of titanium at elevated temperatures, thefull potential of these alloys is seldom reached. While the attainmentof operating temperatures in the 750 F. range would be advantageous inmany instances, and this temperature level is attainable strengthwise intitanium alloy systems, the current practical limitations arenevertheless on the order of 650 F. Inasmuch as the materialdeficiencies are largely manifestations of a high surface reactivity, itis evident that if the full potential of the titanium alloys is to bereached, surface protection or a coating will be required.

While a number of coating techniques for the titanium alloys have beenproposed, none are completely satisfactory in solving the surfacereactivity problem with respect to the highly stressed alloy componentsin the 750900 F.-temperature range. Most often the particular prior arttechnique has been directed toward providing improvement to oneparticular property of titanium, such as galling or corrosionresistance. In most instances, however, the specific improvement isattained only at the expense of some other characteristic of the coatedalloy. In the case of the metallic or ceramic coatings wherein theprocess results in diffusion or in an alloying reaction with thesubstrate metal, an intermetallic is generally formed and it is wellknown that the majority of the intermetallics formed with titanium arecharacteristically brittle.

The establishment of corrosion protection of various metals has alsobeen provided in the past through the use of certain paints or enamels,including the pigmented silicone resins. However, as utilized, thesepaints, while effective in promoting corrosion resistance without theformation of brittle intermetallics or other, detrimental compounds withthe basis metal, do not usually possess either the necessary adherenceto the substrate or sufiicient temperature capabilities to result insubstantial utility in the highly stressed high-performanceapplications. The silicone resins, for example, are usually limitedinterms of temperature to 650 F.

What is really neededin terms of surface protection for the titaniumalloys is a coating whichis chemically bonded to the substratethrough achemical reaction therewith but which does not lead-to detrimentallosses in mechanical properties as a result of such interaction,particularly through the formation of brittle intermetallics, and whichis capable of extending the operatingconditionsfor the titanium alloysto the 750950 F. temperaturerange.

SUMMARY OF THE INVENTION It is the primaryobject of the presentinvention to provide an adherentcoating for titanium and its alloyswhich is capable of extending the usable operating range of certain ofthese alloys to temperatures of 750-900 F. or higher.

It is a further object of the invention to provide asmooth oxidation andcorrosion resistant coating for the titanium alloys, particularly asutilizedin connection with gas turbine engine components, which does notresult in the formation of detrimental compounds or phases with thebasis metal.

The above objects and other objects and advantages: are achieved by aprocess which results in an impervious coating which consists of thedecomposition products of an aluminumpigmented silicone resin, formed insitu on thesurface to be protected, by partial oxidation thereof at hightemperature and chemical reaction with the basis metal. Inasmuch astitaniurn participates in the reaction and thus contributes to its ownprotection in the formation of what is'thought to be analuminum-titanium-carbon-silicon-oxygenquasi-organic polymer, thecoating achieved is chemically bonded to the substrate. Furthermore, itstemperature capabilities are synergistically extended to temperatures of900 F. or higher.

The process involves coating the surface to be protected with a mixturecomprising a nonreactive, polydimethyl siloxane binder and aluminum in aweight ratio of about l/1-3/ 1, preferably about 2, together withsufficient solvents for convenient application, and subsequentlyreacting and partially oxidizing the coating at temperatures in excessof 650 F.

In the particularly preferred process, the alloy is cleaned anddeoxidiaed; coated with a mixture comprising a nonreactive polydimethylsiloxane binder and aluminum in a weight ratio of about 2; cured at450-550 F. for at least 15 minutes; and reacted for about 1-8 hours at atemperature of 6S0-950 F. in an inert atmosphere containing about 10-40percent oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a graph comparing thesmooth fatigue capability of the titanium8 weight percent aluminum --1percent molybdenum-l percent vanadium alloy in both an uncoatedcondition and as coated according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Stress corrosion of thetitanium alloys is dependent upon three primary factors: the presence ofcorroding elements, elevated temperatures and applied or residualtensile stresses. In gas turbine engines, titanium parts are oftenstressed in tension above their so-called s/c level. Similarly, elevatedtemperatures are unavoidably present in engine operation. Therefore,under these conditions, either the corroding elements must be removed orparts must be otherwise protected before the engine operatingtemperatures are reached. Various schemes are and have been employed forreducing the presence of corroding elements, and various techniques,such as peening, stress relief by heat treatment and design techniques,are employed for reducing the effects of stress. However, it is apparentthat metallurgical immunity to stress corrosion is far from a realityand that with todays designs utilizing available titanium alloys,coatings of some type are required for surface protection if maximumutility of these alloys is to be achieved. And, previously discussed,there is presently no coating available which affords the desiredsurface protection.

Concurrently with improvements in terms of stress corrosion, a trulysuitable coating will also provide protection against oxidation andafford improvement with respects to those phenomena associated withsurface related effects, including fatigue and creep stability, up tothe limits of the titanium alloys in terms of their physical properties.

It has been found that a properly balanced formulation of particularsilicone resins with appropriate metallic pigments, particularlyaluminum flakes, typically with compatible diluents when applied to andcured in the right thickness on a clean, nonscaled titanium alloysurface, may be partially oxidized, employing proper controlledconditions, to provide a complex matrix ofaluminum-titanium-silicon-carbonand oxygen with sufficient integrity,adherence and imperviousness to inhibit oxidative and/or corrosiveaction on the titanium alloy-basis metal which thereby becomes both aparticipant in and beneficiary of the coatings system. Synergistically,whereas the silicone paints are normally temperature limited at about650 F., the present coating affords protection to the substrate totemperatures of 750900 F. or higher. Further, inasmuch as the titaniumalloy is a participant in the reaction a chemical bond with thesubstrate is thus achieved.

The measured and observed beneficial effects of the coating and theapparent interaction of the basis metal as a partial contributor to itsown protection, and the absence of any detectable diffusion ormetallurgical (solid solution) reaction between the substrate and thecoating, is in contrast to and thus not comparable to the other siliconor silicon-aluminum coatings whose bond is achieved during or by vapordeposition or by other coating techniques which depend uponmetallurgical diffusion or physical bonding for adequate adhesion. Thechemical ionic bonding of the present invention, as opposed to thediffused coating for titanium, does not result in any dilution ofmechanical properties such as fatigue resistance.

The two titanium alloys currently receiving the major attention arethose identified in the industry as MST-811 which consists of, byweight, 8 percent aluminum, 1 percent molybdenum, l percent vanadium,balance titanium; and the Ti 6-4 alloy which consists of, by weight, 6percent aluminum, 4 percent vanadium, balance titanium.

In the most preferred coating process, the coating mixture consists of,by weight, 57.8 percent nonreactive dimethyl siloxanes, such as DowCorning 805 and 806 silicone resins; 28.9 percent aluminum pigment,lining grade or flake aluminum; and 13.3 percent inert ingredients suchas xylene. This basic formulation is available from Product TechniquesCo. as PT 332.

This mixture is further diluted just prior to use with solvents such asxylene or toluene in a ratio of 4 parts pigmented resin to 1 partthinner and applied by conventional paint spraying techniques to apredetermined thickness to yield a finished coating of 0.00050.004 inchor more preferably 00015-00025 inch. it must be applied to the titaniumafter cleaning and deoxidation and before a substantial reoxidation Theabove process results in a smooth, adherent coating of good appearanceand handling durability which significantly increases the useful life,strength and reliability of the highstrength titanium alloys such asMST-8l 1 in nonbeaning applications to 'stress and temperature levelsappreciably higher than those currently established.

FAT GU ESTING.

. STRE S 908395. 9

Salted Creep In these tests, the time to failure at 900 F. of peened andcoated specimens were established in excess of 150 hours at the stressrupture stress of 65,000 p.s.i., compared with a 150- hour life at55,000 p.s.i. for nonpeened coated articles and less than 55,000 p.s.i.for peened only or noncoated specimens. h ssla a isswsmar sdin T blet VV coated TABLE I.-SALTED CREEP TESTING Salted creep (900 F.) Tensile(RT) Time to Elonga- Creep, failure, Strength, tion, per- Priortreatment percent hours p.s.i. cent 9. 6 0. 5 15. 9 0. 3 2. 7 3. 4 2. 53. 9 0.4 11.0 0. 6 12. 0 2. 2 41. 5 1. 6 29. 0 (0 14. 4 0. 9 10. 7 0. 8510. 1 1. 25 7. 14 3. 65 1. 43 1. 5 1, 37 0.9 42 14. 2 B7 34. 8 8. 12 0.6 8. 7 0. 3 0) 000 8. 2 4. 7 000 9. t) 2. 9 .000 5. 4 105. 6 000 2. 949. 4 ,000 2,12 150.0 122,300 2. 7 Do.. 55, 000 2. 20 150.0 120, 800 2.0Glass bead peened 5N2/15N2 plus Alsilicone complex coating 85. 000 7. 862. 7 (0 0) D 85. 000 8. 78 2. 3 75, 000 13. 40 45. 1 75,000 10.60 32.2000 9. 80 150.0 130, 900 2. 7 65 000 6. 85 150.0 136, 200 6. 0 1. 76150. 0 144, 200 17. 3 1. 71 150. 0 141. 800 4. 0

r {jailed in creep.

has occurred inasmuch as a chemical reaction with the basis metal isrequired. Typically, the surface is chemically and/or abrasively cleanedand coated within 1 hour, or preferably 10 minutes of cleaning.

The as-sprayed coating is then allowed to air-dry usually for no lessthan 15 minutes or no longer than 8 hours whereat it is cured in airfree of contaminants, particularly dust, moisture, halogens, and otherreactive materials. The preferred curing conditions consist of 15minutes to 2 hours at a temperature of 450550 F.

Following the curing step, partial oxidation and decomposition resultingin the formation of the finished coating is ac- Bent Beam This testentailed stressing of l-inch wideS-inch long sheet stock specimens ofapproximately 0.050-inch thickness in the range of plastic deformation.At 900 F. it was found that the 65 present coating provided completeprotection against sea salt complished by heat treating the coating at650950 F. for

i cident to the coating.

V Coated first-stage compressor blades subjected to 7 engine testingunder 800 F. inlet condition showed that,

although the coating was abraded away from those areas subjected tosevere abrasive conditions, the coating remained intact on all areas notsubject to severe abrasion.

The durability of the coating is both a function of the abrasiveconditions and the temperatures to which the coating is exposed. Forlong-term protection, exposures to temperatures up to about 750 F. wouldbe specified although significant improvements are evident up to atleast about 900 F. Even with a 750 F. temperature limitation, however,the operating range of the alloy is increased by 75-l00 F. overconventional systems, with no detrimental effects on mechanicalproperties, and wide temperature excursions are readily accommodated.

While the invention has been described in detail with reference tocertain examples and preferred embodiments, these are intended to beillustrative only. lt will be understood that the invention is not to belimited to the exact details described, for obvious modifications willoccur to those skilled in the art.

What is claimed is:

l. The method of imparting surface protection to titanium and thetitanium alloys which comprises:

cleaning and deoxidizing the surface to be protected;

prior to a substantial reoxidation, coating the surface to a i ns.9about 04 h Fi a ixtur -1.

2. The method according to claim 1 wherein:

the final heat treatment is conducted at a temperature of about 650-950F.

3. The method according to claim 2 wherein:

the final heat treatment is conducted in an inert atmosphere containingabout 10-40 volume percent oxygen.

4. The method according to claim 2 wherein:

the final heat treatment is conducted in a nitrogen atmospherecontaining about 10-40 volume percent oxygen.

5. The method according to claim 4 wherein:

the weight ratio of dimethyl siloxane to aluminum is about 6. The methodaccording to claim 4 wherein: curing is effected at a temperature ofabout 450-$50 F. 7. The method of coating the high-strength titaniumalloys which comprises:

cleaning and deoxidizing the surface to be protected;

within about 1 hour, coating the surface to a thickness of about00005-0004 inch with a pigmented silicone resin mixture consistingessentially of a nonreactive dimethyl siloxane and aluminum pigment in aweight ratio of about 2, and sufficient inert dispersant to permitconvenient application;

within about 8 hours, curing the coating in an uncontaminated airatmosphere at 450-550 F. for at least 15 minutes;

and heat treating the coating for about 1-8 hours at a temperature of650-950 F. in an inert atmosphere containing about 10-40 volume percentoxygen.

UNITED STATES PATENT OFFICE '(5/6 CERTIFICATE OF CORRECTION Patent No.3,640,778 Dated uary 8, 19 7-2 Inventor) Jules P. Winfree, Herbert E.Todd, Charles C. McComas It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 1, column 5, line 24 0.005 should read 0.0005

Signed and sealed this hth day of July 1972.

(SEAL) Attest:

EIWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. The method according to claim 1 wherein: the final heat treatment isconducted at a temperature of about 650*-950* F.
 3. The method accordingto claim 2 wherein: the final heat treatment is conducted in an inertatmosphere containing about 10-40 volume percent oxygen.
 4. The methodaccording to claim 2 wherein: the final heat treatment is conducted in anitrogen atmosphere containing about 10-40 volume percent oxygen.
 5. Themethod according to claim 4 wherein: the weight ratio of dimethylsiloxane to aluminum is about
 2. 6. The method according to claim 4wherein: curing is effected at a temperature of about 450*-550* F. 7.The method of coating the high-strength titanium alloys which comprises:cleaning and deoxidizing the surface to be protected; within about 1hour, coating the surface to a thickness of about 0.0005- 0.004 inchwith a pigmented silicone resin mixture consisting essentially of anonreactive dimethyl siloxane and aluminum pigment in a weight ratio ofabout 2, and sufficient inert dispersant to permit convenientapplication; within about 8 hours, curing the coating in anuncontaminated air atmosphere at 450*-550* F. for at least 15 minutes;and heat treating the coating for about 1-8 hours at a temperature of650*-950* F. in an inert atmosphere containing about 10-40 volumepercent oxygen.